New Variable Stiffness Joint to Provide Constant Force in an Arm Rehabilitation Device

Abstract Rehabilitation robotics offers a promising approach to enhancing recovery in patients with musculoskeletal or cerebrovascular injuries. This article presents the development and validation of a novel variable stiffness joint (VSJ) that replaces large motors to generate resistive forces, thereby facilitating controlled recovery of muscle tone. For a revolute joint mechanism, we show that a constant output force can be maintained by adjusting stiffness, as demonstrated through simulation studies. Further validation was conducted through prototype experiments with volunteer participants, confirming that spring-length control enables modulation of joint stiffness and maintains a constant force at the link's end regardless of the rotation angle or trajectory. This work highlights an effective actuation method capable of delivering personalized and responsive rehabilitation therapy.